The present invention relates to a mobile radio transmission system and to a mobile station for use in the system.
Such a system typically comprises a plurality of base stations situated at respective geographical locations to define a plurality of service areas within an overall region and a plurality of mobile stations capable of communicating by two-way radio with each base station when in communication range of that base station. Each base station comprises the entirety of transmitters and receivers operated by a trunking system controller (TSC) at any one site or service area. The communication link established is two-frequency simplex, that is one frequency is allocated to forward transmission from a base station to a mobile station and a second frequency is allocated for return transmission from a mobile station to a base station. Blocks of radio channels chosen from an overall band of channels are allocated to each base station. In the case of trunked radio, the base stations of a region may be connected by way of land line to a control station (CS) or may be connected as a mesh with the base stations interconnected with each other. There may be two or more regions in which case each region has its own CS which controls a plurality of associated base stations. The CSs are connected together by land lines.
In a known such system each mobile station is arranged when operative to register with a base station of which it is within communication range and thereby with the corresponding service area, and to respond to it being no longer within communication range of a base station serving a service area with which it is currently registered by registering with another base station of which it is within such range (if any) and thereby with the corresponding service area. The base station responds to the registration request by storing the identity of the service area corresponding to the relevant base station in that one of its location records which corresponds to the registering mobile station. The registration procedure is carried out so that if it is desired to call a particular mobile station it is known in which service area that mobile station is currently located; the CS arranges that the call is directed to the or each base station which serves the relevant service area, which base station in turn calls the relevant mobile station. An advantage of a registration procedure is that if there was no provision for registration then all the base stations of the system would have to call the particular mobile station, which would impose a considerable overhead on the transmission capacity of the system. However, a disadvantage with this known registration procedure is that if a mobile station is roaming near a border between two or more service areas so that it frequently moves between areas it will frequently change its registration from one service area to another, again imposing a considerable overhead on the communication channel capacity of the system.
European Patent Specification EP-A2-0260763 discloses a multiple registration method aimed at reducing the transmission overhead associated with the above described system. This multiple registration method enables the TSC to store a list of up to n registrations for each mobile. The mobiles each have storage facilities for storing the same list of registrations as are stored by the TSC in respect of itself. As a mobile enters a new service area it automatically registers with the area's base station thus updating the list of registration stored by the mobile and its associated TSC. By storing these registrations an enlarged service area is created. If a TSC wishes to contact a mobile it refers to the list of registrations which it is storing for that mobile and initially transmits a signal to the base stations which are registered, either successively, beginning with say the most recent registration, or simultaneously. If this is not successful then the TSC tries the other base stations. Provision is made for reordering the list on the basis that the channel number and frequencies of the service area in which a mobile last made a call are at the top of the list. In the event of the list of registrations not being updated and/or a call not being made to or received from a mobile after a set period then it is possible for registrations to be cancelled one at a time thereby reducing the size of the enlarged service area. While it is possible for all the registrations to be cancelled, it is preferred that at least one registration of a service area remain to prevent a flood of registrations at the start of a working day. If, at the start of a working day, a mobile is not registered in the service area in which it is currently located it has to arrange to immediately register. This method is quite workable but does have a drawback in that it will not necessarily optimise the usage of radio channels.
This drawback will be better understood by considering a hypothetical case of a circular geographical region of about 128 km with 7 base stations serving it. Obviously hilly areas tend to mask some mobiles from some base stations, and the signal strength obtained by a mobile from a base station varies inversely with the distance to the base station. The propagation map allows a generous margin of signal strength so that most mobiles can "see" most base stations.
The frequencies currently used in Great Britain for trunked private mobile radio systems are around 200 MHz, which are very penetrating and give good coverage, especially since the mobiles are extremely sensitive.
The power and number of radio channels fitted per base station are carefully chosen to correspond to the expected activity local to that base station.
The radio protocol used, for example a subset of MPT 1327-A Signalling Standard for Trunked Private Land Mobile Radio Systems issued by the Department of Trade and Industry, London 1988, effectively requires the mobiles to hold on to a registration as long as they can. The intention is, of course, to reduce the overhead of registration signalling on the network, which supplies no calls to the user.
Furthermore, the MPT 1327 protocol has a good error-correction layer, and so a significant number of on-air bits can be lost before any particular signalling packet fails. In consequence a mobile will not attempt to re-register on another base station unless the packet-level error rate on his present one is high. However on the fringe of the coverage area this can lead to a poor grade of service both in terms of speech signal-to-noise and in call set-up failure rate.
The result of the above set of circumstances is to change the radio topology of the network from being a number of largely isolated cells with little overlap, resulting in the registration map largely corresponding to the geographical position of the mobile, to one in which the cells very greatly overlap, causing the registration map to be largely random. Each base station has a number of mobiles registered on it proportional to its radio coverage, and these mobiles are distributed over the part of the region in which this can be "seen". Each mobile has only a small chance of being registered on the base station where it is most likely for its calls to be satisfied by that base station alone, which base station may be regarded as the mobile's home site or home service area.
The effect of this is that a high percentage of calls say 80% are intersite calls, using 2 radio traffic channels per call instead of one, the system capacity is almost halved, and the number of calls each base station must support bears little relation to the carefully estimated local population on which its size and power were based. This means that a base station gets a load proportional to the size of its radio coverage, not the number of channels fitted. This has the potential of grossly overloading the smaller service areas. Creating more service areas within the region by adding more base stations actually makes things worse, since it means that each mobile's chance of being registered on its home site is proportionally reduced.